The present invention relates to a method of bonding biologically active (hereafter called "bioactive") glass coatings to metal substrates, particularly medical prostheses, and to a metal substrate coated with bioactive glass.
Bioactive glasses for coating of metal substrates suitable for prostheses or surgical implants are known in the art, e.g. U.S. Pat. No. 4,234,972. A method is disclosed allowing use of glasses and metals which do not have closely matched coefficients of thermal expansion.
Matching coefficients of thermal expansion are required when conventional coating methods are used requiring high firing temperatures. On subsequent cooling, thermomechanical stress in the glass layer is reduced when thermal expansion coefficients match so avoiding cracks in the glass coating.
It is an object of the invention to increase the bond strength between the glass and the metal by developing a chemical bond between the glass and the metal on firing the glass-coated metal substrate at high temperature. In view of the high firing temperatures matching thermal expansion coefficients are essential.